The decision to not charge for literature was likely the biggest financial blunder the WTS ever committed. Because, once that horse was out of the barn, it wasn't coming back. They could have continued charging and just charged sales tax and remitted like a normal business. But, they likely mistakenly thought that the voluntary donation "arrangement" would generate sufficient funds because JWs would continue "donating" the same amount for literature as they used to. Remember, you paid for the literature upfront and then sold it to the public. Of course, there were some that donated whatever they received but most didn't. What happened is that without the specific charge associated, JWs stopped giving that amount and thought their normal contributions were covering the cost of the literature.
For example, if the rags were 50 cents each and you got 5 sets a month,so that's $5 to the literature servant. Then say you donated $10 per month to the worldwide work. OK that's $15 total. What the WTS thought would happen was that the WWW donation would move to $15. What happened was it stayed at $10.